1. Field of the Invention
The present invention generally relates to a base station for communicating with mobile stations that move within a service area including plural cells, and relates to a transmission power control method.
2. Description of the Related Art
When a conventional cellular mobile communication system forms a service area, especially in the wireless access scheme like W-CDMA in which the same frequencies are shared with adjacent cells, it is necessary to efficiently use apparatuses, sites, aggregated lines and frequencies while maximizing communication quality and system capacity.
For this purpose, before constructing a wireless base station, desk design using computer simulations and the like is performed to design the position and configuration of the base station, and wireless parameters such as a transmission power level of a downlink common control channel transmitted from the base station by predicting a service area and traffic. After constructing the wireless base station, it is necessary to periodically perform an actual survey of the service area, communication quality, and system capacity, and it is necessary to perform maintenance. As mentioned above, for constructing a wireless base station, it is necessary to perform complicated cell system designing/evaluation work (refer to non-patent document 1 and non-patent document 2, for example).
On the other hand, in a future cellular mobile communication system, since the band of occupied frequencies in the wireless access scheme is widened due to increase of throughput, that is, transmission speed per one user, there are tendencies for high frequency bands in which unoccupied frequencies remain to be used. Accordingly, propagation loss including diffraction loss due to land objects largely increases and transmission power of the system is limited due to requirements of apparatuses and safety, so that the allowable maximum cell radius decreases, wherein the diffraction loss due to land objects is a loss caused when a radio wave arrives at a position near the surface of the earth from the top of a building.
Therefore, it is becoming difficult to systematically form the service area using macro/micro cells by installing antennas at high positions such as a steel tower, housetop, mountaintop and the like that has been generally performed. Thus, there is a possibility that service area formation (cell downsizing) based on street micro-cells and pico-cells in which antennas are installed in higher density at positions lower than buildings will be mainstream. Service area development by cell downsizing is effective for overcoming the propagation loss and for increasing user throughput and system capacity. However, since the number of base stations largely increases, cell system design/evaluation become further complicated.
In addition, it becomes difficult to systematically form conventional hexagonal cells due to the cell downsizing. Therefore, a base station is required that can be adapted to the surrounding environment even when base stations are installed without planning and unevenly, and that can operate while avoiding unnecessary interference in cooperation with surrounding base stations. For achieving this purpose, an autonomous distributed cell forming method is proposed (refer to non-patent document 3, for example). According to the autonomous distributed cell forming method, in the micro-cell environment, the base station sets wireless parameters to form a cell autonomously in a distributed manner in order to simplify system designing/evaluation.
More precisely, in this method, the transmission power of the pilot channel that is one of the downlink common control channel sent from a base station is initialized, in every base station, to the same maximum transmission power level that can be set. Next, each mobile station in a handover area compares a received level of the channel with a required received level, and reports the comparison result to the base station as a control power amount, wherein the received level is a measured result such as received power, ratio of received power to interference power, and ratio of received power to total received power within the spreading band. Next, the base station simply averages the reported values from many mobile stations to determine optimum control power amount. Accordingly, the transmission power of the pilot channel is controlled so that the cell is formed.
However, in this case, it is necessary to predetermine the required received level for each cell environment comprising such as standard deviation of shadowing, correlation between base stations, cell radius and the like, by performing computer simulation or experiment such that an out-of-range ratio of mobile stations becomes equal to or less than a predetermined value. In addition, the control of the transmission power of the pilot channel is performed only once. Thus, when a base station is newly installed, it is necessary to initialize the transmission power of the pilot channel of all base stations again so that the service is interrupted.
[Non-patent document 1] Hayashi, et. al., IEICE General Conference 2000, B-5-81, “Study on transmission power distribution to downlink common control channel in W-CDMA scheme”, March 2000.
[Non-patent document 2] Mori, et. al., IEICE General Conference 2000, B-5-34, “W-CDMA area evaluation experiment using received quality measurement system”, March 2001.
[Non-patent document 3] Mogi, et. al., IEICE Society Conference, B-5-105, “autonomous distributed cell forming method using surrounding base station information in CDMA cellular system”, September 2003.
However, above-mentioned conventional techniques have the following problems.
In the street micro-cells and the pico-cells that are smaller than the micro-cells, deviation of traffic distribution among cells is large. Therefore, in the case when there are large variations in cell radii, efficient cell formation cannot be necessarily performed by the conventional cell forming method for controlling transmission power of the pilot channel using only the received level of the pilot channel.
This problem is described with reference to FIG. 1. The mobile station determines a base station to be connected to based on the received level of the pilot channel. For example, when the mobile station receives pilot channels from plural base stations, a base station corresponding to the highest received level is called a main branch, a base station corresponding to a received level that differs from the level of the main branch by equal to or less than a predetermined value is called a major branch, and each of the other base stations is called a minor branch.
A major branch may exist or may not exit depending on the position of the mobile station. For example, when the mobile station exists near the center of the cell, namely, near a particular base station, received level of the main branch is very high, so that the differences between the received level of the main branch and other received levels become greater than the predetermined value. Thus, a major branch does not exist, and every other base station is a minor branch. On the other hand, when the mobile station exists near a border of cells, namely near a place located nearly equidistant from plural base stations, the difference between the received level of the main branch and a received level of another base station becomes equal to or less than the predetermined value so that a major branch exists. In the cellular mobile communication system using CDMA, the major branch is an object of soft handover (simultaneous connection).
In the following, a case where a mobile station exists on a line connecting a first base station and a second base station is considered with reference to FIG. 1. In the case shown in FIG. 1, the first base station forms a cell that is extremely large due to transmission power control of the pilot channel for forming the cell, and the second base station forms a cell that has a smaller radius and that is adjacent to the large cell.
The mobile station exists at a position nearer to the second base station than the first base station. However, since the received level of the pilot channel transmitted by the first base station is greater than the received level of the pilot channel transmitted by the second base station, the cell of the first base station becomes a main branch for the mobile station, and the cell of the second base station becomes a minor branch. Thus, the second base station is out of a soft-handover area. In this case, the mobile station accepts transmission power control from the first base station, so that the mobile station sets transmission power so as to obtain necessary communication quality for uplink to the first base station.
Since the first base station is further than the second base station from the viewpoint of the mobile station, the propagation loss is large so that the mobile station performs transmission with excessive power from the viewpoint of the second base station. Therefore, the second base station receives large interference from the mobile station.
If the second base station is a major branch that is an object of soft-handover, the communication quality of the uplink line needs to be satisfied in either of the first base station or the second base station, and the transmission power of the mobile station can be controlled by the second base station. Therefore, the above-mentioned interference does not occur. However, when difference between radii of cells adjacent to each other is large, there is a high probability that a base station that is not an object of soft-handover but is nearer than a connecting base station exists like the case shown in FIG. 1.
In addition, when a cell is formed only on the basis of the received level of the pilot channel without considering load status of the base station, the transmission power of the pilot channel may be increased even when the total transmission power of the base station is exceeding a limiting value (high workload state). As a result, the coverage is enlarged so that there occurs a problem in that call originating right is provided to many mobile stations and congestion may occur in downlink.
Further, increase of transmission power of the pilot channel in the high workload status causes increase of interference to communication channels. In the CDMA scheme in which the transmission power control is applied to communication channels, there is a problem in that downlink congestion occurs when the transmission power is increased for keeping communication quality of the communication channel.
Therefore, it is important to realize a mechanism for controlling excessive interference input to uplinks while considering coverage and downlink workload (congestion) status.